Method of making soldered joints

ABSTRACT

A process for making a soldered joint between two members, one of which may comprise aluminum. The ends of the members to be joined are formed with sufficient clearance so that they can easily be telescoped, but preferably so that telescoping of the members forms a tight fit at a tapered neck to temporarily, frictionally retain the members in assembled relation. Prior to telescoping, either one or both of the members is coated with a layer of zinc solder so that after telescoping a layer of solder is disposed in the clearance space between the members in the region of overlap. The members are then partially submerged in a molten zinc solder bath and ultrasonic energy is applied only to the molten solder. The solder between the members is heated to its melting temperature which causes it to flow and break the surface tension at the interface with the solder bath so that solder from the solder bath flows into and completely fills the clearance space between the joint members. The ultrasonic energy can then penetrate into the region between the two members to clean the surfaces of the members without any direct contact between the ultrasonic transducer and the members being joined. When the members are withdrawn from the molten solder, the solder solidifies between them to form a permanent joint. The porcess is especially adapted for joining return bends of multi-row heat exchangers because the ends of the heat exchangers can be moved through the molten solder bath without direct contact with the ultrasonic transducers.

United States Patent [191 Greever METHOD OF MAKING SOLDERED JOINTS [75]Inventor: 'James E. Greever, Dewitt, NY.

[73] Assignee: Carrier Corporation, Syracuse, NY. [221' Filed: Aug. 8,1972 [21] Appl. No.: 278,846

[52] 1.8. CI 29/l57.3 R, 29/157.6, 29/4701,

29/492, 29/503, 228/1, 228/36, 228/40 [51] Int. Cl. B2ld 53/02, 823p15/26 [58] Field of Search 29/4701, 492, 503,

Primary Examiner J. Spencer Overholser Assistant Examiner-Robert J.Craig Attorney-J. Raymond Curtin et a1.

[ 7] ABSTRACT A process for making a soldered joint between two .1 Sept.25, 1973 members, one of which may comprise aluminum. The ends of themembers to be joined are formed with sufficient clearance so that theycan easily be telescoped,

' but preferably so that telescoping of the members forms a tight fit ata tapered neck to temporarily, frictionally retain the members inassembled relation. Prior to telescoping, either one or both of themembers is coated with a layer of zinc solder so that after telescopinga layer of solder is disposed in the clearance space between the membersin the region of overlap. The members are then partially submerged in amolten zinc solder bath and ultrasonic energy is applied only to themolten solder. The solder between the members is heated to its meltingtemperature which causes it to flow and break the surface tension at theinterface with the solder bath so that solder from the solder bath flowsinto and completely fills the clearance space between the joint members.The ultrasonic energy can then penetrate into the region between the twomembers to clean the surfaces of the members without any direct contactbetween the ultrasonic transducer and the members being joined. When themembers are withdrawn from the molten solder, the solder solidifies between them to form a permanent joint. The porcess is especially adaptedfor joining return bends of multirow heat exchangers because the ends ofthe heat exchangers can be moved through the molten solder bath withoutdirect contact with the ultrasonic transducers.

12 Claims, 9 Drawing Figures PATENTEDSEPZSIQTS III 8 FIG.6

' METHOD OF MAKING SOLDERED JOINTS BACKGROUND OF THE INVENTION madeunless the oxide is first cleaned off and not allowed to re-form whilethe joint is being soldered. Further, the zinc solder employed informing an aluminum joint has much higher surface tension than typicalcopper brazing material and therefore does not flow readily into smallspaces.

One method of obtaining good adherence between the solder coating andthe aluminum is to apply ultrasonic energy to the aluminum members whilethey are submerged in a bath of molten solder. This technique can beused for making a bell and spigot joint between the aluminum members bydirectly vibrating the telescoped members in a molten zinc solder bath.Unfortunately, however, this process is excessively cumbersome forproduction of multiple row heat exchanger return bend joints becauseeach of the return bend members must be individually brought intocontact with an ultrasonic transducer in order to form a satisfactoryjoint. The process, consequently, requires excessively expensive toolingfor use on a production ba- SIS.

Another technique which has been found to be successful for joiningaluminum members involves forming the members to be joined in a bell andspigot relationship such that the members havean interference fit witheach other. The precoated members are then simultaneously heated andforced into one another to form a joint. While this process has beensuccessful in joining individual tubes to each other, it has seriousshortcomings for a production operation. Complex fixturing is requiredbecause of the extremely accurate alignment necessary to simultaneouslyforce a plurality of interfering return bend members into the ends ofthe heat exchange tubes of the heat exchanger. Also, it is difficult andexpensive to manufacture both the belled ends of the heat exchange tubesand the spigot members with sufficiently great dimensional accuracy toprovide the required interference fit. It is therefore desirable toemploy large clearances in the joint for economical production. However,the use of large clearances has previously resulted in unsatisfactoryjoints when using prior aluminum soldering processes unless ultrasonicenergy was directly applied to the joint as explained above,

Prior attempts at joining multi-row aluminum retumbend members of heatexchangers by applying the ultrasonic energydirectly to the solder bathrather than to the individual members. have been generally unsuccessfulbecause of the difficulty in transferring sufficient energy into thesolder to provide the required cleaning of oxides from all of themembers. Furthermore, it is undesirable to use high intensity ultrasonicenergy because it adversely affects'the life of the ultrasonic horn andit erodes and destroys the metal walls of the members being joined.

Applicant has discovered that a principal reason for the failure ofprior attempts to join aluminum members submerged in molten zinc solderby application of ultrasonic energy to the solder alone is probably dueto the inability of the ultrasonic energy to adequately penetrate intothe region between the members being joined to provide the requiredcleaning action. This, in turn, is believed to be due to the inabilityof the solder to flow into and completely fill the area between thejoint members because of the existence of a relatively high surfacetension at the interface of the joint with the molten zinc solder.

SUMMARY OF THE INVENTION In accordance with this invention, it has beenfound that telescoped aluminum members having a substantial clearancecan be joined by applying ultrasonic energy to only the molten zincsolder bath if a layer of zinc solder is present initially between themembers being joined. The solder layer is heated in the solder bath andis caused to melt in a manner to break the surface tension with themolten solder, thereby enabling solder from the bath to flow into andfill the space between the members being joined. This, in turn, enablesthe ultrasonic energy to penetrate into the space between the members toeffectively clean their surfaces so that a good solder bond is achieved.

One, or preferably both, of the members to be joined is coated with alayer of adherent solder and the members are telescoped so that thesolder layer lies between them. The members are then partially submergedin molten solder with the outer member opening downwardly, andultrasonic energy is applied to only the molten solder. When the solderbetween the members being joined melts, it comingles with the moltensolder in the bath and breaks the surface tension at the region of thejoint entrance, thereby allowing the clearance space between the membersto become completely occupied with liquid solder by capillary action.When this region fills with solder, the ultrasonic energy penetratesbetween the members with sufficient intensity to clean the innersurfaces of the members and provide an adherent coating of solder whichforms a permanent joint when the members are withdrawn from the solderbath and the solder is cooled.

This process is especially adapted for commercial production of aluminummulti-row heat exchangers having aluminum return bends. Preferably, thereturn bends are inserted into belled ends of the tubes of the heatexchanger, so that the return bends are temporarily, frictionallysecured to a tapered neck portion on the heat exchange tube bells duringsubsequent processing. Since the members being joined need not bedirectly contacted by the ultrasonic transducers, all of the return bendjoints of a multiple row heat exchanger can be made by simply dippingthe assembled heat exchanger into the solder bath and removing it in acontinuous operation.

BRIEF DESCRIPTION OF THE DRAWINGS FlGfl is a fragmentary view partiallyin cross section of a typical heat exchanger having return bend jointsmade in accordance with this invention;

FIG. 2 is a cross sectional view illustrating the coating of a returnbend with solder;

FIG. 3 is a cross sectional view taken substantially on line III-III ofFIG. 2;

FIG. 4 is a view illustrating the assembly of a return bend with theheat exchange tubes;

FIG. 5 is a cross sectional view illustrating a heat exchanger passingthrough a molten solder bath;

FIG. 6 is a cross sectional view taken substantially on line Vl-VI ofFIG. 5;

FIG. 7 illustrates the initial condition of assembled joint membersafter being submerged in the solder bath;

FIG. 8 illustrates the condition of the joint members after heating inthe solder bath; and

FIG. 9 is a drawing similar to FIG. 7 illustrating a modified embodimentof the joint members.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, there isshown a heat exchanger 10 having a plurality of rows of parallel heatexchange tube members 11. In the heat exchanger illustrated, a pluralityof plate fins 12 are disposed between a pair of tube sheets 13 (only oneof which is shown) to form a plate fin heat exchanger. A plurality ofreturn bend members 14 are inserted in bells 15 formed in the end ofheat exchange tubes 11. The heat exchanger illustrated is of the typewherein return bends l4 lie in a plurality of spaced parallel planes.While a two row deep heat exchanger is illustrated, various othermultirow heat exchangers with parallel or non-parallel return bends maybe made in accordance with the method described herein.

Referring now to FIG. 2, there is illustrated a solder pot having aquantity of molten zinc solder 21 therein, which forms a molten solderbath. A pair of ultrasonic transducers 22, including horns, which may beof any conventional type, supply ultrasonic energy to solder 21. Areturn bend 14 to be coated with solder is dipped in solder bath 21, asbest shown in FIG. 3, and ultrasonic energy is simultaneously impartedto the solder bath. The application of ultrasonic energy to a solderbath causes an erosion which removes the oxide from the exterior ofreturn bend 14 and causes fusion to occur as the result of directcontact between the solder and the bare metal of the return bend. Whenreturn bend 14 is removed from the solder bath, an adherent coating ofsolder 25 quickly solidifies as the return bend is cooled by contactwith ambient air. The layer 25 of solder is preferably about 0.001 inchthick and may be applied to either the spigot or bell members orpreferably to both. To obtain a good joint, it is necessary for anadherent solder coating to be provided on at least one and preferablyboth of the members being joined so that it lies between the membersduring processing in solder pct 30. Actually, it is preferred to coatboth the bell members on their interior surfaces and the return bends ontheir exterior surfaces with solder for best results in obtaining a goodreturn bend joint.

As shown in FIG. 4, return bend 14, having a layer of zinc solder 25thereon, is inserted into the belled ends of heat exchange tubes 11.Preferably, heat exchange tubes 11 are formed with bells 15, having atapered neck portion 27, having an inner diameter at some point which isless than the outer diameter of return bend 14. The tapered neck portion27 is accordingly formed so as to cause a tight fit with the end ofreturn bend member 14 at some point in the bell when the return bend isinserted into the ends of heat exchange tubes. It is desirable that theparent metal of return bend member 14 form a close fit with the metal ofbell 15 in the region of neck 27, irrespective of solder coating 25, sothat the return bend will be temporarily mechanically secured to theheat exchange tubes by a friction fit after insertion therein tofacilitate subsequent processing. While it is desirable for the membersto fit tightly so they are retained in assembled relation, it is alsonecessary that the fit is not so tight as to prevent venting of thejoint during the soldering process, so the solder can enter the joint bycapillary action. Likewise, substantial clearance space is requiredbetween the members over most of their area of overlap to provide aspace for solder to flow in forming the finished joint. The sides of thebell preferably have a substantial initial clearance with the returnbend of about 0.01 inch along most of its length, which typically may bereduced to about 0.009 inch by solder layer 25, so as to facilitateinsertion of the return bends into the ends of the heat exchange tubes.

The temporarily assembled joint members are then lowered into a secondpot 30 containing molten zinc solder to heat the adhering solder layer.FIG. 7 illustrates the condition of a return bend joint after it isinitially plunged into the second solder pot. It will be seen that adistinct meniscus 38 is present at the interface of clearance space 39with the molten solder. This figure also illustrates the engagementbetween the end of return member 14 and tapered portion 27 of bell 15which temporarily frictionally retains the members in engagement butallows venting of clearance space 39 to the interior of heat exchangetube 11.

Solder layer 25 extends an axial distance on the end of return bend 14greater than the distance of overlap between the members so that itextends below the end of bell 15. After a period of time,joint members14 and 15 and adherent solder coating 25 are heated to at least themelting point of the solder coating. As solder layer 25 begins to melt,it wets and comingles with the molten zinc solder in the region belowthe end of bell 15 to break the surface tension at meniscus 38 whichnormally prevents the solder from flowing into clearance space 39between the joint members. When this wetting has occurred, the moltenzinc flows freely up into the clearance space between the joint membersas illustrated in FIG. 8.

After solder has flowed into the clearance space, the joint is broughtinto an ultrasonic field within the solder bath. The solder completelyfilling the clearance space between the joint members provides a pathfor the ultrasonic energy to reach and clean the interior surfaces ofthe joint members. Consequently, the interior surface of bell 15 and theexterior surface of return bend 14 are scrubbed and cleanedultrasonically so that the zinc solder can fuse with the aluminum jointmembers.

FIG. 9 illustrates a slightly modified bell construction wherein bell 15has a more moderately tapered throat region 40 and an outwardly flaringlip portion 41. This construction is essentially adapted for use inconnection with automated assembly of the components because of the easewith which the return bend members may be inserted within the valve.

As shown in FIGS. 5 and 6, solder pot 30, having molten zinc solder 31therein, may be provided with a specially configured array of ultrasonictransducers and horns 32. Ultrasonic transducers having horns 32 exotherby a distance which will permit the movement of 5 return bends past thehorns without contact therewith.

. Heat exchanger having return bend members 14 termporarily securedthereto by the tight fit with bell 26 are moved along a conveyor belt sothat the return bends assume a path similar to that designated bynumeral 36. Path 36 lies in a plane substantially parallel with theplanes of return bend members 14. The return bend members dip intosolder pot 30 in a heating reg'ion 35 where they are brought up to atleast the melting temperature of solder coating 35. The amount ofheating is determined by the length of time the return bends reside inthe heating region before reaching the region of the ultrasonic horns.Heating region 35 is preferably of enlarged volume, compared to thesoldering region, to increase its heat storage capacity and improvetemperature uniformity of the bath.

The heat exchanger is preferably aligned so that return bends 14 passbetween adjacent ultrasonic transducers 32 in spaced relation therewith.As return bends 14 are moved to a position adjacent to the homsoftransducers 32, a burst of ultrasonic energy is supplied directly to themolten solder 31 by. the transducers so that the return bends lie in anultrasonic field. The prior heatingof solder layer 25 causes the pretinlayer of solder on the return bends to melt and flow so that the surfacetension of the solderin the solder pot is broken at the entrance to thejoint clearance between the members. Soldertherefore flows andcompletely fills the clearance space between the joint members bycapillary action. Since the space between the joint members is nowcompletely filled with solder, the ultrasonic en ergy applied to thesolder is able to penetrate from the outer bell member into this regionto ultrasonically scrub the inner joint surfaces being bonded. Priorultrasonic processes for soldering aluminum tube joints are believed tohave been unsatisfactory because the surface tension of the molten zincsolder was so great that it prevented the region between the jointmembers from completely filling with solder. Consequently, insufficientultrasonic energy penetrated the void space between the members becausethere was insufficient solder available in the joint area to conduct theenergy to achieve the required cleaning of the inner joint surfaces.

The heat exchanger is thereafter raised out of solder pot 30 and thejoint cooled by contact with the ambient air. Cooling of the jointrapidly solidifies a firmly adherent coating of zinc solder between thejoint members, thereby forming a permanent joint of very high quality.

Many heat exchangers are of the multiple row type wherein there are fouror-six rows of heat exchange tubes in the direction of air flow acrossthe heat exchanger. With such heat exchangers, it is often we ferredpractice to employ multiple refrigerant circuits within the heatexchangers. Consequently, in such coils the return bends are not alwaysall parallel with each other, but a number of them are often placedtransversely to the plane of others. Consequently, the heat exchangercannot be passed between rows of parallel transducers in the solderpotas illustrated in the drawing. in such cases, it has been found that itis possible to mount the ultrasonic transducers and horns on the bottomof the solder pot and to focus sufficient energy at the proper locationin the molten solder to provide good return bend joints. The returnbends are assembled with the heat exchanger tubes as previouslydescribed. The heat exchanger'is then lowered into the molten solder andleft there for sufficient time to melt the solder between the returnbend and the heat exchange tubes, so that the surface tension is brokenat the interface with the molten solder. An ultrasonic energy field isthen applied to the solder to clean the surfaces of the tubes and formthe final joint as described before. 7

In order to form a good return bend joint, the depth of submergence ofthe return bend in the molten solder is important. The outer member ofthe joint, having the greater diameter, which receives the other member,must be disposed in a downward direction in the solder bath.Furthennore, the outer member must be only partially submerged to'adepth less than the extent of the'penetration of the inner member intothe outer member so that the surface of the molten solder is below thetop of the inner member. It has been found that if the surface of thesolder is above the top of the inner member, there is a tendency for themolten solder to flow up into the clearance space between the jointmembers and to overflow the top of the inner member, which results inexcess solder accumulating in the interior of the return bend.

For a number of reasons, it is preferred that the ends of the heatexchange tubes be enlarged to form a bell which receives the straightcylindrical sides of thereturn bend. First, it is easy to bell the endsof the heat exchange tube members in the process of expanding the heatexchange tube into contact with the fins. Secondly, by belling the endsof the heat exchange tube members and inserting the return bendstherein, only a portion of the ends of the heat exchange tubes need besubmerged in the molten solder so that the fin portion of the tubes arenot coated. In addition, a bending operation is all that is required toform the return bend from tubing. Also, the short return bends do notlend themselves as easily to having bells formed thereon as do the heatexchange tubes.

The tapered neck of the outer bell member can be a relatively sharpbevel as illustrated in the drawing, or the whole bell member can have agradually tapered neck along the axial extent of the bell. In eithercase, the return bend can easily be assembled into the high clearancebell so as to secure it by a friction fit for subsequent lowering intothe solder. As previously noted, this invention avoids the problemsinherent in joining two members having a close fit along their entirelength, and therefore, overcomes a significant fixturing problem in aproduction operation.

While this invention finds its principal application in the joining ofaluminum members, it will be apparent that the process may also beemployed to solder various other metals to aluminum. Accordingly, theinvention may be otherwise embodied within the scope of the followingclaims.

I claim:

In a method of making a tubular joint between the ends of two tubularmembers, at least one of which comprises aluminum, the steps including:

' A. forming the end of said members to be joined of a size to enablethe ends to be telescoped with a clearance space therebetween;

B. applying an adherent coating of solidified solder to the end of atleast one of said members;

C. inserting the end of one of said members into the end of the other ofsaid members, so that the coating of solidified solder is present in theclearance space between the overlapping portion of the ends of the innerand outer telescoped members;

D. positioning the telescoped members so that the open end of the outerof the members is disposed downwardly;

E. submerging the downwardly disposed open end of the outer of the twotelescoped members into molten solder to heat the assembled members andthe coating of solder therebetween to at least the melting point of thesolder, and to thereby break the surface tension of the solder at theregion of the joint by melting and flowing of the solidified soldercoating, thereby causing molten solder to flow into and fill the regionbetween the two members;

F. directly applying ultrasonic energy to only the molten solder whilethe inserted ends of the members lie in the ultrasonic energy field inthe molten solder, whereby the ultrasonic energy penetrates into theclearance space between the joint members to clean the inner surfaces ofthe members; and

G. withdrawing the members from the molten solder,

and cooling them to solidify the solder in the clearance space betweenthe members to fonn a joint therebetween.

2. A method as defined in claim 1 wherein the step of applying coatingof solder to the end of one of said members includes submerging the endof that member in molten solder; and applying ultrasonic energy to themolten solder while the end of the member is submerged therein.

3. A method as defined in claim 1 wherein the step of inserting the endof one of the members into the end of the other of the members includesaxially applying force of sufficient magnitude to the members tofrictionally retain the members in assembled relation during subsequentprocessing.

4. A method as defined in claim 1 wherein the step of forming the endsof said members includes forming a bell with a tapering portion on oneof the members, said tapering portion having an interior dimensionsmaller than the corresponding exterior dimension of the other of saidmembers; and wherein the step of inserting the end of the other of saidmembers into the bell formed on said one member includes forcing themembers together so as to frictionally retain the members in assembledrelation during subsequent processmg.

5. A method as defined in claim 1 wherein the step of applying anadherent coating of solder to at least one of said members includesapplying said adherent coating to the end of the inner of said membersan axial distance greater than the area of overlap between the memberswhen they are assembled so that the solder coating extends downwardlyfrom the downwardly disposed open end of the outer member when theassembled members are submerged in molten solder, to thereby afford aregion of substantial area for comin gling of the heated coating ofsolder and the molten solder surrounding it, facilitating breaking ofthe surface tension of the molten solder at its interface with theclearance space between the joint members.

6. A method as claimed in claim 1 wherein the step of submerging theinserted ends of the members into molten solder includes disposing thebottom end of the outer of the inserted members below the surface of themolten solder while the upper end of the inner of the inserted memberslies above the level of molten solder.

7. In a method of joining a plurality of return bend members to the endsof a plurality of heat exchange tube members of a heat exchanger,wherein the ends of said tube members are formed of sufficient size toreceive the ends of said return bend members with a clearance spacetherebetween, the steps including:

A. applying an adherent coating of solder to the ends of at least one ofsaid members;

B. inserting and temporarily securing the ends of the return bendmembers into the ends of the heat exchange tube members with theadherent layer of solder disposed in the clearance space between theoverlapping ends of said members, thereby providing a plurality ofreturn bends assembled with the heat exchanger;

C. positioning the heat exchanger so that ends of the heat exchange tubemembers open downwardly;

D. lowering the ends of the heat exchange tube members, having thereturn bend members inserted therein, into molten solder to heat theadherent layer of solder to its melting point, whereby the melting ofthe adherent coating of solder breaks the surface tension of the moltensolder at the region of the inserted ends of the members so that moltensolder is enabled to flow into the clearance space between the insertedends of the members;

E. applying ultrasonic energy to only the molten solder while theinserted ends of the members lie in the ultrasonic energy field in themolten solder, whereby the ultrasonic energy penetrates into theclearance space between the joint members to clean the inner surfacesthereof; and

F. withdrawing the heat exchanger from the molten solder and cooling itto solidify solder in the clearance space between the heat exchange tubeand the return bend members and form a permanent joint therebetween.

8. A method as defined in claim 7 including the step of forming a bellon the ends of said heat exchange tube members with a tapering portion,said tapering portion having a portion having an interior dimensionsmaller than the corresponding exterior dimension of the other of saidmembers; and wherein the step of inserting the end of the return bendmembers into the bells formed on said heat exchange tube member includesforcing the members together so as to frictionally retain the members inassembled relation during subsequent processmg.

9. A method as defined in claim 7 wherein the step of applying anadherent coating of solder to at least one of said members includesapplying said adherent coating to the end of the inner of said membersan axial distance greater than the area of overlap between the memberswhen they are assembled so that the solder coating extends downwardlyfrom the downwardly disposed open end of the outer member when theassembled members are submerged in molten solder, to thereby afford aregion of substantial area for comingling of the heated coating ofsolder and the molten solder surrounding it, facilitating breaking ofthe surface tension of the molten solder at its interface with theclearance space between the joint members.

10. In a method of joining a plurality of return bend members to theends of a plurality of heat exchange tube members of a heat exchangerhaving multiple parallel rows of heat exchange tubes, wherein the endsof said tube members are formed of sufficient size to receive the endsof said return bend members with a clearance space therebetween, thesteps including:

A. applying an adherent coating of solder to the ends of at least one ofsaid members;

B. inserting and temporarily securing the ends of the return bendmembers into the ends of the heat exchange tube members, with theadherent layer of solder disposed in the clearance space between theoverlapping ends of said members, thereby providing a plurality ofreturn bends assembled with the heat exchanger which lie in parallelplanes;

C. positioning the heat exchanger so that ends of the heat exchange tubemembers open downwardly; D. lowering the ends of the heat exchange tubemembers, having the return bend members inserted therein, into moltensolder to heat the adherent layer of solder to its melting point,whereby the melting of the adherent coating of solder breaks the surfacetension of the molten solder at the region of the inserted ends of themembers so that molten solder is enabled to flow into the clearancespace between the inserted ends of the members;

E; passing the ends of the heat exchange tube members, having returnbend members inserted therein, along a path parallel with the planes ofsaid pluralvity of return bends, past a plurality of ultrasonictransducers disposed in the molten solder, so that the return bends passbetween adjacent ultrasonic transducers in spaced relation therewith;

F. applying ultrasonic energy to only the molten sollid der while theinserted ends of the members lie in an ultrasonic field in the moltensolder, whereby the ultrasonic energy penetrates into the clearancespace to clean the inner surfaces of the members; and

G. withdrawing the heat exchanger from the molten solder and cooling itto solidify solder in the clearance space between the heat exchange tubeand return bend members and form a permanent joint therebetween.

11. A method as defined in claim 10 including the step of forming a bellon the ends of said heat exchange tube members with a tapering portion,said tapering portion having a portion having an interior dimensionsmaller than the corresponding exterior dimension of the other of saidmembers; and wherein the step of inserting the end of the return bendmembers into the bells formed on said heat exchange tube member includesforcing the members together so as to frictionally retain the members inassembled relation during subsequent processing.

12. A method as defined in claim 10 wherein the step of applying anadherent coating of solder to at least one of said members includesapplying said adherent coating to the end of the inner of said membersan axial distance greater than the area of overlap between the memberswhen they are assembled so that the solder coating extends downwardlyfrom the downwardly disposed open end of the outer member when theassembled members are submerged in molten solder, to thereby afford aregion of substantial area for comingling of the heated coating ofsolder and the molten solder surrounding it, facilitating breaking ofthe surface tension of the molten solder at its interface with theclearance space between the joint members.

1. In a method of making a tubular joint between the ends of two tubularmembers, at least one of which comprises aluminum, the steps including:A. forming the end of said members to be joined of a size to enable theends to be telescoped with a clearance space therebetween; B. applyingan adherent coating of solidified solder to the end of at least one ofsaid members; C. inserting the end of one of said members into the endof the other of said members, so that the coating of solidified solderis present in the clearance space between the overlapping portion of theends of the inner and outer telescoped members; D. positioning thetelescoped members so that the open end of the outer of the members isdisposed downwardly; E. submerging the downwardly disposed open end ofthe outer of the two telescoped members into molten solder to heat theassembled members and the coating of solder therebetween to at least themelting point of the solder, and to thereby break the surface tension ofthe solder at the region of the joint by melting and flowing of thesolidified solder coating, thereby causing molten solder to flow intoand fill the region between the two members; F. directly applyingultrasonic energy to only the molten solder while the inserted ends ofthe members lie in the ultrasonic energy field in the molten solder,whereby the ultrasonic energy penetrates into the clearance spacebetween the joint members to clean the inner surfaces of the members;and G. withdrawing thE members from the molten solder, and cooling themto solidify the solder in the clearance space between the members toform a joint therebetween.
 2. A method as defined in claim 1 wherein thestep of applying coating of solder to the end of one of said membersincludes submerging the end of that member in molten solder; andapplying ultrasonic energy to the molten solder while the end of themember is submerged therein.
 3. A method as defined in claim 1 whereinthe step of inserting the end of one of the members into the end of theother of the members includes axially applying force of sufficientmagnitude to the members to frictionally retain the members in assembledrelation during subsequent processing.
 4. A method as defined in claim 1wherein the step of forming the ends of said members includes forming abell with a tapering portion on one of the members, said taperingportion having an interior dimension smaller than the correspondingexterior dimension of the other of said members; and wherein the step ofinserting the end of the other of said members into the bell formed onsaid one member includes forcing the members together so as tofrictionally retain the members in assembled relation during subsequentprocessing.
 5. A method as defined in claim 1 wherein the step ofapplying an adherent coating of solder to at least one of said membersincludes applying said adherent coating to the end of the inner of saidmembers an axial distance greater than the area of overlap between themembers when they are assembled so that the solder coating extendsdownwardly from the downwardly disposed open end of the outer memberwhen the assembled members are submerged in molten solder, to therebyafford a region of substantial area for comingling of the heated coatingof solder and the molten solder surrounding it, facilitating breaking ofthe surface tension of the molten solder at its interface with theclearance space between the joint members.
 6. A method as claimed inclaim 1 wherein the step of submerging the inserted ends of the membersinto molten solder includes disposing the bottom end of the outer of theinserted members below the surface of the molten solder while the upperend of the inner of the inserted members lies above the level of moltensolder.
 7. In a method of joining a plurality of return bend members tothe ends of a plurality of heat exchange tube members of a heatexchanger, wherein the ends of said tube members are formed ofsufficient size to receive the ends of said return bend members with aclearance space therebetween, the steps including: A. applying anadherent coating of solder to the ends of at least one of said members;B. inserting and temporarily securing the ends of the return bendmembers into the ends of the heat exchange tube members with theadherent layer of solder disposed in the clearance space between theoverlapping ends of said members, thereby providing a plurality ofreturn bends assembled with the heat exchanger; C. positioning the heatexchanger so that ends of the heat exchange tube members opendownwardly; D. lowering the ends of the heat exchange tube members,having the return bend members inserted therein, into molten solder toheat the adherent layer of solder to its melting point, whereby themelting of the adherent coating of solder breaks the surface tension ofthe molten solder at the region of the inserted ends of the members sothat molten solder is enabled to flow into the clearance space betweenthe inserted ends of the members; E. applying ultrasonic energy to onlythe molten solder while the inserted ends of the members lie in theultrasonic energy field in the molten solder, whereby the ultrasonicenergy penetrates into the clearance space between the joint members toclean the inner surfaces thereof; and F. withdrawing the heat exchangerfrom the molten solder and cooling it to solidify solder in theclearance space between the heat exchange tube and the Return bendmembers and form a permanent joint therebetween.
 8. A method as definedin claim 7 including the step of forming a bell on the ends of said heatexchange tube members with a tapering portion, said tapering portionhaving a portion having an interior dimension smaller than thecorresponding exterior dimension of the other of said members; andwherein the step of inserting the end of the return bend members intothe bells formed on said heat exchange tube member includes forcing themembers together so as to frictionally retain the members in assembledrelation during subsequent processing.
 9. A method as defined in claim 7wherein the step of applying an adherent coating of solder to at leastone of said members includes applying said adherent coating to the endof the inner of said members an axial distance greater than the area ofoverlap between the members when they are assembled so that the soldercoating extends downwardly from the downwardly disposed open end of theouter member when the assembled members are submerged in molten solder,to thereby afford a region of substantial area for comingling of theheated coating of solder and the molten solder surrounding it,facilitating breaking of the surface tension of the molten solder at itsinterface with the clearance space between the joint members.
 10. In amethod of joining a plurality of return bend members to the ends of aplurality of heat exchange tube members of a heat exchanger havingmultiple parallel rows of heat exchange tubes, wherein the ends of saidtube members are formed of sufficient size to receive the ends of saidreturn bend members with a clearance space therebetween, the stepsincluding: A. applying an adherent coating of solder to the ends of atleast one of said members; B. inserting and temporarily securing theends of the return bend members into the ends of the heat exchange tubemembers, with the adherent layer of solder disposed in the clearancespace between the overlapping ends of said members, thereby providing aplurality of return bends assembled with the heat exchanger which lie inparallel planes; C. positioning the heat exchanger so that ends of theheat exchange tube members open downwardly; D. lowering the ends of theheat exchange tube members, having the return bend members insertedtherein, into molten solder to heat the adherent layer of solder to itsmelting point, whereby the melting of the adherent coating of solderbreaks the surface tension of the molten solder at the region of theinserted ends of the members so that molten solder is enabled to flowinto the clearance space between the inserted ends of the members; E.passing the ends of the heat exchange tube members, having return bendmembers inserted therein, along a path parallel with the planes of saidplurality of return bends, past a plurality of ultrasonic transducersdisposed in the molten solder, so that the return bends pass betweenadjacent ultrasonic transducers in spaced relation therewith; F.applying ultrasonic energy to only the molten solder while the insertedends of the members lie in an ultrasonic field in the molten solder,whereby the ultrasonic energy penetrates into the clearance space toclean the inner surfaces of the members; and G. withdrawing the heatexchanger from the molten solder and cooling it to solidify solder inthe clearance space between the heat exchange tube and return bendmembers and form a permanent joint therebetween.
 11. A method as definedin claim 10 including the step of forming a bell on the ends of saidheat exchange tube members with a tapering portion, said taperingportion having a portion having an interior dimension smaller than thecorresponding exterior dimension of the other of said members; andwherein the step of inserting the end of the return bend members intothe bells formed on said heat exchange tube member includes forcing themembers together so as to frictionally retain the memberS in assembledrelation during subsequent processing.
 12. A method as defined in claim10 wherein the step of applying an adherent coating of solder to atleast one of said members includes applying said adherent coating to theend of the inner of said members an axial distance greater than the areaof overlap between the members when they are assembled so that thesolder coating extends downwardly from the downwardly disposed open endof the outer member when the assembled members are submerged in moltensolder, to thereby afford a region of substantial area for comingling ofthe heated coating of solder and the molten solder surrounding it,facilitating breaking of the surface tension of the molten solder at itsinterface with the clearance space between the joint members.